E-4 is not AF1 however it can be in time of national emergency and often flies in close proximity to bot AF1 747's.
General Mattis has also used the E-4 for overseas flights.
SpaceX Launches Secret Zuma Mission for US Government
https:// www.space.com/38826-spacex-launches-secret-zuma-mission-lands-rocket.html
"But the main goal of tonight's flight was getting Zuma aloft, so the spacecraft can start going about its business. Just what that business may be is unclear; little has been revealed about the payload.
We do know that aerospace and defense company Northrop Grumman procured Zuma's launch atop a Falcon 9 for the U.S. government — but we don't know which agency will operate the satellite, or if its mission is civilian or military."